1. Field of the Invention
The present invention relates to a delta-sigma modulator and a delta-sigma modulation method that apply delta-sigma (ΔΣ) modulation to an analog input signal or a digital input signal having a plurality of bits to generate a 1-bit digital signal.
2. Description of the Related Art
A ΔΣ modulated 1-bit audio signal has a format including a very high sampling frequency and a short data word length (for example, a sampling frequency of 64 times 44.1 kHz and a data word length of 1-bit) as compared with a format of data used in conventional digital audio (for example, a sampling frequency of 44.1 kHz and a data word length of 16 bits). The ΔΣ modulated 1-bit audio signal has an advantage of a wide transmissible frequency range. Even with the 1-bit signal, the ΔΣ modulation makes it possible to obtain a wide dynamic range in an audio range of low frequencies relative to the 64 times oversampling frequency. Taking advantage of this feature, the ΔΣ modulation can be applied to a recorder and data transmission for high-quality sound.
A ΔΣ modulation circuit itself is not an especially novel technique; the circuit is commonly used within a conventional A/D converter or the like, because the circuit configuration is suitable for integration into an IC and the circuit can achieve a high A/D conversion precision relatively easily. The ΔΣ modulated signal can be restored to an analog audio signal by passing the ΔΣ modulated signal through a simple analog low-pass filter.
Conventionally, in a ΔΣ modulator, when a signal with a certain constant frequency is input, the same signal is repeated within the ΔΣ modulator because the ΔΣ modulator has a feedback system, with the result that a distorted or unstable signal is output.
FIG. 1 shows a configuration of a conventionally-known ΔΣ modulator 100. The ΔΣ modulator 100 is a fifth-order ΔΣ modulator having five integrators 109, 111, 113, 115, and 117. Each of the integrators includes an adder and a delayer. For example, the integrator 109 includes an adder 109a and a delayer 109b. 
An input signal input through an input terminal 101 is supplied to coefficient calculators 102, 103, 104, 105, 106, and 107 at a time. The coefficient calculators 102, 103, 104, 105, 106, and 107 are calculators for performing calculation control like gain control or filter control for an input signal and have feedforward coefficients ff0, ff1, ff2, ff3, ff4, and ff5, respectively.
The coefficient calculators 102, 103, 104, 105, and 106 perform calculation for the feedforward coefficients ff0, ff1, ff2, ff3, and ff4. The obtained respective calculation outputs are supplied to the adders 109a, 111a, 113a, 115a, and 117a of the integrators 109, 111, 113, 115, and 117. The coefficient calculator 107 performs calculation for the feedforward coefficient ff5. The obtained calculation output is supplied to the adder 118, and added to the integrated output of the integrator 117. The added output of the adder 118 is supplied to a quantizer 119.
The quantizer 119 applies quantization to the added output and supplies an output terminal 125 with the quantized output and, at the same time, feeds back the quantized output to coefficient calculators 120, 121, 122, 123 and 124 to be described later. The quantizer 119 generates a 1-bit output signal by applying quantization to the added output while referring to a threshold value of 0, which is constant at all times with respect to time. That is, the quantizer 119 applies quantization to the added output, which is an input signal, by determining two value levels using the threshold of 0, that is, determining whether the input signal is 0 or more, or less than 0.
The coefficient calculators 120, 121, 122, 123, and 124 hold feedback coefficients fb0, fb1, fb2, fb3, and fb4, used when the quantized output is fed back to the integrators 109, 111, 113, 115, and 117. The respective calculation outputs from the coefficient calculators 120, 121, 122, 123, and 124 are supplied to adders 108, 110, 112, 114, and 116, which are provided in front of the integrators 109, 111, 113, 115, and 117, and then fed back to the respective integrators.
In the case of, for example, attenuation control or gain control like fade-in or fade-out, the feedforward coefficients ff0, ff1, ff2, ff3, ff4, and ff5 of the coefficient calculators 102, 103, 104, 105, 106, and 107 are determined by a not-shown controller and the gain of input signals is controlled.
Further, in the case of filter control that passes or blocks only a desired frequency band, the feedforward coefficients ff0, ff1, ff2, ff3, ff4, and ff5 of the coefficient calculators 102, 103, 104, 105, 106, and 107, and the feedback coefficients fb0, fb1, fb2, fb3, and fb4 of the coefficient calculators 120, 121, 122, 123 and 124 are determined by the controller and input signals are subjected to filter control.
In the fifth-order ΔΣ modulator 100 shown in FIG. 1, when a signal with a constant repetition frequency, called a fixed pattern, is input, or when a 0 level signal is input at the time when the input of the ΔΣ modulator is opened, a distorted or unstable signal is output, because the ΔΣ modulator 100 has a configuration that feeds back a difference between the output of the integrator and the quantized value thereof.
FIG. 2 shows the frequency analysis result of a 1-bit output signal of 128 fs (fs=44.1 kHz) in the case where a signal of a fixed pattern is input. FIG. 3 shows the frequency analysis result of a 1-bit output signal of 128 fs (fs=44.1 kHz) in the case where a 0 level signal is input as an input signal. As can be seen from FIGS. 2 and 3, the configuration that feeds back a difference between the output of the integrator and the quantized value thereof when a signal of a certain fixed pattern or 0 level signal is input makes noise shaping, which is one of the features of the ΔΣ modulator, ineffective, with the result that a distorted or unstable signal is output.
To avoid this, in the conventional ΔΣ modulator, a random noise signal is input through the input side component of the ΔΣ modulator (for example, through the adder 108 in the case of the ΔΣ modulator 100 of FIG. 1). Alternatively, a random component is inserted at the front stage of the quantization performed in the ΔΣ modulator to eliminate the fixed pattern.
However, the insertion of a random component may degrade the signal characteristics. To cope with this problem, there is available a method that calculates an adequate amount of the random component and inputs it equivalently to the quantizer, as disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2002-314425 (Patent Application No. 2001-157947) by the present applicant. The delta-sigma modulator disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2002-314425 controls a threshold level to be referred to in quantization processing of the quantizer in a variable manner with respect to a time axis in order to input the calculated adequate amount of random component equivalently to the quantizer.
FIG. 4 shows a configuration example of the above mentioned delta-sigma modulator 130 according to Jpn. Pat. Appln. Laid-Open Publication No. 2002-314425 that uses the adequate amount of random component. The ΔΣ modulator 130 is a fifth-order ΔΣ modulator having five integrators 139, 141, 143, 145, and 147. Each of the integrators includes an adder and a delayer. For example, the integrator 139 includes an adder 139a and a delayer 139b. 
In the delta-sigma modulator 130, a multi-channel audio signal represented by using a 1-bit audio data from CH 1 to CH 6 is reproduced from, for example, an optical disc 131 like a Super Audio CD (SA-CD) by a not-shown disc reproduction apparatus and supplied to coefficient calculators 132, 133, 134, 135, 136, and 137 at a time. The coefficient calculators 132, 133, 134, 135, 136, and 137 are calculators for performing calculation control like gain control, filter control, or mix control for the respective 6-channel multi-channel audio signals and have coefficients b0, b1, b2, b3, b4, and b5 for the respective calculation processes. In the case of the mix control, the coefficients b0, b1, b2, b3, b4, and b5 are determined by a controller to be described later and the respective multi-channel signals are mixed.
The coefficient calculators 132, 133, 134, 135, and 136 perform calculation for the coefficients b0, b1, b2, b3, and b4. The obtained respective calculation outputs are supplied to the adders 139a, 141a, 143a, 145a, and 147a of the integrators 139, 141, 143, 145, and 147. The coefficient calculator 137 performs calculation for the coefficient b5. The obtained calculation output is supplied to the adder 148, and added to the integrated output of the integrator 147. The added output of the adder 148 is supplied to an adder 149 provided in front of a quantizer 150.
Further, a random noise signal is supplied to the adder 149. The random noise signal has been set in an adequate amount by a random noise generator 152 that generates a random noise signal like a dither signal. Thus, the adder 149 adds an adequate amount of the random noise signal to the added output of the adder 148 and supplies the quantizer 150 with the added output including the random noise signal.
The random noise signal generator 152 supplies the quantizer 150 with the random noise signal whose gain has been set in an adequate amount based on the integrated output of the last stage integrator 147 through the adder 149. That is, a gain calculator 151 calculates the gain based on the integrated output of the last stage integrator 147 and sets in the random noise generator 152.
The gain calculator 151 calculates a gain by which a random noise signal Rn is multiplied such that the absolute value of the random noise signal Rn becomes less than or equal to a variable threshold Δq (|Rn|≦Δq) which is based on the amplitude of the signal inside the last stage integrator 147.
The adder 149 adds the random noise signal Rn whose gain has been adjusted as described above to the integrated output of the last stage integrator 147 and, the quantizer 150 quantizes the added output signal.
The adequate amount of gain that has been calculated by the gain calculator 151 is equal to the threshold ±Δq of the quantizer, the threshold being variable with respect to time. The variable threshold Δq is calculated based on the amplitude of the signal generated in the last stage integrator 147. More concretely, the variable threshold Δq is obtained as SαDend which is calculated by multiplying the maximum value Dend of the amplitude of the signal generated in the last stage integrator 147 by a predetermined constant Sα.
FIG. 5 shows a conceptual view of a quantizer using the threshold variable with respect to time. The adequate amount of ±Δq is, for example, not more than 75 with respect to ±1.0 of the quantizer. When an amount that exceeds that value is input, delta-sigma modulation becomes unstable, making it impossible to suppress distortion.
The quantizer 150 applies quantization to the added output to which the adequate amount of random noise signal has been added and supplies an output terminal 159 with the quantized output as well as feeds back the quantized output to coefficient calculators 153, 154, 155, 156, and 157 to be described later.
The coefficient calculators 153, 154, 155, 156, and 157 hold feedback coefficients a0, a1, a2, a3, and a4, used when the quantized output is fed back to the integrators 139, 141, 143, 145, and 147. The respective calculation outputs from the coefficient calculators 153, 154, 155, 156, and 157 are supplied to adders 138, 140, 142, 144, and 146, which are provided in front of the integrators 139, 141, 143, 145, and 147, and then fed back to the respective integrators. The ΔΣ modulator 130 of FIG. 4 having the configuration described above can apply quantization to an input signal at an optimal variable threshold level.